I guess the problem may come from the integrated BEC: Battery Eliminator circuit. The driver will cut the motor power when the "main" battery lower than a "specific" voltage, so that the remaining power can let the R/C receiver and the servo so save the model from danger! But there must be something that can make the driver "wake up" again when the battery become a acceptable voltage.

How to wake it up again?(without switch off and then restart the receiver/uC again)

Stanley